Zend/CodeGenerator/Php/Docblock/Tag.php require_once's Zend/CodeGenerator/Php/Docblock/Tag/Param.php, which in turn require_once's Zend/CodeGenerator/Php/Docblock/Tag.php. That second require_once of Zend/CodeGenerator/Php/Docblock/Tag.php isn't performed since the file is already included, but because it hasn't been fully processed by PHP yet the class Zend_CodeGenerator_Php_Docblock_Tag doesn't exist, so PHP throws a fatal error when trying to define class Zend_CodeGenerator_Php_Docblock_Tag_Param.